1. Field of the Invention
The present invention relates to a process for forming a deposition film useful for semiconductor devices, photosensitive devices for solar batteries and electrophotography, and electronic devices such as optical input sensors for optical image input-output devices.
2. Related Background Art
Heretofore, individually appropriate processes for forming deposition films have been used for forming crystal functional films such as semiconductor films, insulating films, photoconductive films, magnetic films or metal films, based on desired physical properties and applications.
For example, in the formation of silicon crystal deposition films, a thermal CVD method, a hydrogen reduction method, a molecular beam epitaxial method (MBE), a sputtering method, a plasma CVD method, a photo CVD method etc. have been heretofore used.
Among these methods, the thermal CVD method and the hydrogen reduction method have been widely used in the semiconductor industry, but these two methods require so high a deposition temperature that they can fully meet neither a desired lowering of temperature in the semiconductor process nor formation of crystalline deposition films on low melting point substrates such as glass.
The molecular beam epitaxial method (MBE) is capable of forming crystal deposition films at a relatively low temperature. However, but owing to the deposition on substrates by a direct chemical absorption of raw material molecules, the deposition is readily influenced by contamination in the reaction chamber or impurities on the surfaces of substrates. Accordingly, an ultra high vacuum or a clear surface is required for the deposition.
Thus, the MBE method is not always suitable for mass-production from the viewpoint of apparatus cost, maintenance or output.
The plasma CVD method and the photo CVD method have been recently regarded as promising, and have been studied as processes for forming crystal deposition films of high quality at a lower temperature.
However, for example, studies of formation of silicon deposition films by a plasma CVD method or by a photo CVD method, have indicated that, the reaction process is more complicated than that of the conventional CVD method, and its reaction mechanism has not been fully clarified yet.
There are many deposition film formation parameters such as substrate temperature, flow rate of introduced gases and their ratios, pressure in the reaction space during the formation of deposition films, high frequency power, electrode structure for high frequency power input, structure of reaction chamber, venting rate from the reaction chamber, light intensity and wavelength, photoabsorbancy and heat conductivity of the substrate, etc. which are complicatedly interrelated with one another.
Thus, the conditions for forming deposition films sometimes become unstable or at times substantially impossible to carry out the deposition while keeping all the parameters in a state set to the optimum conditions.
Owing to these problems, the plasma CVD method and the photo CVD method are not always satisfactory as a method for forming good crystalline deposition films of large area with a good reproducibility and at a low cost.
On the other hand, another process for forming good crystalline deposition films of large area at a low temperature with a good reproducibility, i.e., a process for forming a crystalline deposition film on a substrate by separately introducing gaseous starting materials for forming a deposition film and a gaseous oxidizing agent into a reaction space and then chemically contacting them, (which will be referred to as chemical deposition process below,) is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 62-96675.
In the above-mentioned process, the reaction to form a deposition film depends substantially on pressures and flow rate ratio of the gaseous starting material and the gaseous oxidizing agent, the substrate temperature, etc.
That is, the process has less restrictions to the controllability of deposition process based on complicated deposition parameters of the conventional process for forming a crystalline deposition film at a low temperature, such as correlations between the flow rates of introduced gases or the pressure in the reaction space and the plasma state, the presence of side reaction by excitation of various chemical species, or correlation between the decomposition of gaseous starting materials by the wavelength or intensity of irradiation light and the reaction on the substrate surface, etc. than in the above-mentioned plasma CVD process. Thus, the chemical deposition process can form a crystalline deposition film of large area with a good uniformity and a good reproducibility.
However, in the above-mentioned chemical deposition process, such a range for conditions, i.e. pressure, flow rate ratio and temperature of a gaseous starting material and a gaseous oxidizing agent, the possibility that the formed deposition film will be in a crystalline form rather than in an amorphous form, is relatively narrow. That is, once a deposition film is formed under the crystallizable condition, the quality of the crystalline deposition film is invariably set. This fact shows that although the chemical deposition process has a good controllability of deposition process by the pressure, flow rate ratios, temperature, etc. of the gaseous starting material and the gaseous oxidizing agent, a crystalline deposition film of required good quality cannot always be obtained within the condition range. Therefore, an improvement of the chemical deposition process by introducing new deposition parameters has been in demand.